The invention relates to filtration systems for removing solids from liquids, and more particularly to an improved filter cake compression apparatus for leaf-type filter systems, useful to compress the cake against the filter surface so that its volume is reduced and it can be easily removed.
Filter cake compression apparatus is shown in U.S. Pat. Nos. 3,708,072 and 3,814,257. The apparatus of these patents utilizes flexible diaphragms resiliently mounted upstream of the perforate filtering surfaces of the filter leaves, so that when the filter cake builds up to the diaphragm, a pressure drop occurs between the cake and the upstream side of the diaphragm so that the diaphragm moves toward the leaf to compress the cake. The diaphragms were generally stretched by connecting tension members inside hoops connected to stationary supports.
Several problems were inherent in these compression systems involving non-movable diaphragm supports. During compression, the supporting springs for the diaphragms tended to pull the diaphragm away from the leaf, thereby diminishing the compressing action being exerted by hydraulic forces. The diminished compression was particularly acute toward the outside periphery of the leaf, where the diaphragm supporting springs acted, with the result that the cake would be wetter and less compressed near the periphery of the leaf. The center area would be the driest and most compressed.
Another problem with these cake compression systems actuated wholly by hydraulic pressure differences stemmed from the fact that not all filtering surfaces build up cake at the same rate. When many filter cakes had reached the diaphragm and were compressed, others had not yet built up to the corresponding diaphragm, and this difference could occur on two sides of the same leaf. Thus, when enough filter cakes had been compressed that little filtration was occurring and cleaning was necessary, some cakes remained wet and uncompressed.